1. Field of the Invention
This invention generally relates to a method of fabricating a solar cell, and in particular to a method of fabricating a differential doped solar cell.
2. Description of the Related Art
In recent years, new forms of renewable energy are of much interest due to problems, such as rising oil prices, global warming, exhaustion of fossil energy, nuclear waste disposal and site selection involved in construction of a new power plant. Among others, research and development into solar cells, which directly converts solar energy into electric energy and is a pollution-free energy, has actively progressed.
The solar cell design in widespread use today has a p-n junction formed near the front surface (that surface which receives the light) which creates an electron flow as light energy is absorbed in the cell. The conventional solar cell design has two electrical contacts formed on the front and rear sides, respectively. In a typical photovoltaic module these individual solar cells are interconnected electrically in series to increase the voltage in order to provide power for electrical appliances which is driven by a large voltage.
The conventional solar cell fabrication may use a p-type semiconductor substrate and then form a thin n-type semiconductor layer on the p-type semiconductor substrate by a high temperature thermal diffusion. Prior to the high temperature thermal diffusion, a texturing structure is formed on the front surface of the semiconductor substrate and an anti-reflection layer is applied to reduce the reflection of incident light. Next, a screen-printing process is performed. Ag paste and Al paste may be screen-printed and dried on the front surface and the rear surface of the semiconductor substrate according to a predetermined pattern by a screen-printed machine. Next, a co-firing process is performed. The Ag paste and Al paste formed on the front surface and the rear surface of the semiconductor substrate are fired through a high temperature sintering furnace so that the Ag paste and Al paste may form a eutectical structure on the corresponding surface of the semiconductor substrate for ohmic contact. Accordingly, the electrical electrodes may be formed on the surfaces of the semiconductor substrate, and a solar cell with simple structure is completed.
For the cost consideration, in fabrication of solar cell, a p-type semiconductor substrate is doped with boron, particularly heavy doped with boron in order to reduce the sheet resistance of the emitter layer. However, it will produce relative open circuit voltage degradation under illumination (luminous decay). Under illumination, a boron atom may combine two oxygen atoms in the semiconductor substrate to become an occurring position of recombination of carriers, reducing short circuit current and decreasing the efficiency of the solar cell.
The inventor used and determined two kinds of samples, which are different concentration of boron-doped substrates with different sheet resistance in order to confirm that the above phenomenon of luminous decay should increase as the addition amount of boron in the substrate. The results are shown in tables 1 and 2. Table 1 shows data of cell outdoor exposure of single cell using a heavy doped boron substrate with sheet resistance of 0.5˜3.0 Ω/sq (Sample Nos. Pcs-01 to Pcs-04). Table 2 shows data of cell outdoor exposure of single cell using a light doped boron substrate with sheet resistance of 3.0˜6.0 Ω/sq (Sample Nos. Pcs-01 to Pcs-04). In tables 1 and 2, the value of cell outdoor exposure is the photoelectric conversion rate of the samples in unit of percentage (%). The specific value of 5 kWh/M2 in the column of “after exposure” represents an overall luminous amount to which the photoelectric conversion rate reduces most sharply, and beyond the specific value the downward trend of the photoelectric conversion rate will significantly slow down and stabilize. In tables 1 and 2, the last column indicates the luminous decay of single cells with Samples Nos. Pcs-01 to Pcs-04, and in the bottom of the last column indicates average decay rates of 1.47(%) and 0.69(%) from which it is obvious the luminous decay of single cells using the substrate with sheet resistance of 0.5˜3.0 Ω/sq is greater than that of 3.0˜6.0 Ω/sq.
TABLE 1(sheet resistance of 0.5~3.0 Ω/sq)SamplebeforeAfter exposureDecayNo.exposure (%)(5 kWh/M2) (%)rate (%)Pcs-0117.8317.621.18Pcs-0218.0917.871.22Pcs-0318.0517.731.77Pcs-0418.1817.871.71Average decay1.47rate
TABLE 2(sheet resistance of 3.0~6.0 Ω/sq)SamplebeforeAfter exposureDecayNo.exposure (%)(5 kWh/M2) (%)rate (%)Pcs-0118.0217.910.61Pcs-0218.0617.900.89Pcs-0318.0217.960.33Pcs-0418.1417.970.94Average decay0.69rate
Therefore, the inventor conducted researches according to the scientific approach in order to improve and resolve the above drawback, and finally proposed the present invention, which is reasonable and effective.